What was the injury? Are you 100% healed and just nervous about it recurring or is it a chronic issue that can flare-up if aggravated?
Really? Still, after three months of zero leg work? I'm just saying, do an honest self-evaluation to see if this is really the case or if it's a convenient excuse to avoid training legs.
Once a week:
A) Squat variation 3-4x10-12
B) Single-leg exercise 2-3x8-10
C) Hamstring exercise 2-3x10-12
If you're just looking for "maintenance", then any exercise will do. Could vary it each week or keep them the same. The trick is going to be, after those three months, make sure you do bump up the intensity and/or volume and/or frequency.